Filled out the Plugin\Email::sendEmail(). Pulls in the following settings from the phpci config under the heading email_settings:

smtp_address
smtp_port
smtp_username
smtp_password

and

from_address
Adding Builder::getSystemConfig() to provide easy access to config.yml.
Fixing the email plugin so that it retrieves the system config correctly.
Adding email settings to the install command.
Updating the email plugin tests so that getSystemConfig is mocked correctly.